There are a number of factors that can cause two supposedly identical clocks to drift apart or lose synchronization. Other than the need to correct for the time offset between two clocks, there is the need to account or compensate for the effects of oscillator aging and drift. Aging is the systematic change in frequency with time due to internal changes in the oscillator. It is the frequency change with time while factors external to the oscillator (environment, power supply, etc.) are kept constant.
Drift is the systematic change in frequency with time. Drift is due to aging plus changes in the environment and other factors external to the oscillator (drift includes aging). Drift is what one observes in an application (i.e., the particular situation where the oscillator is used). Generally aging, rather than drift, is what one measures during oscillator evaluation and is what one specifies in an oscillator specs or datasheet. In the case of aging, for example, a quartz oscillator changes frequency at an approximately predictable rate, and the resultant deviation over time can be determined.
New high quality ovenized quartz crystal oscillators typically exhibit small, positive frequency drift with time unrelated to external influences (i.e., aging). A significant drop in this aging (frequency change) rate occurs after few weeks of operation at operating temperature. Ultimate aging rate below 0.1 ppb (parts-per-billion) per day are achieved by the highest quality crystals and 1 ppb per day rates are commonplace.
The primary effect of temperature variations is change in the oscillator's frequency. Oven oscillators (e.g., Oven Controlled Crystal Oscillators (OCXOs)) offer the best temperature stability when compared to non-oven crystal oscillators and Temperature Compensated Crystal Oscillators (TCXOs). Non-oven crystal oscillators and TCXOs may drift slowly to a new frequency after the ambient temperature changes since the internal thermal time constant can be fairly long. Though careful oscillator design and manufacturing minimize aging at the time of shipment, aging continues for the life of the oscillator and is affected by the circumstances within the time duration after it is powered off and the storage conditions.
Differences in temperature, the age of the oscillators themselves, material variations in the manufacturing process, contamination, mechanical stress, among other factors, can all affect the quality of synchronization. All of these factors create a need for clock synchronization to allow for two clocks to be aligned when differences occur. The continuous variations of the above factors also explain why the process of synchronization is continuous and not a one-time process. Accurate time (or time-of-day or wall-clock) synchronization now has a wide arrange of applications areas including the electrical utility and smart grids, packet networks, telecom industry, industrial automation, and testing and measurement systems.
Oscillators vary in their ability to maintain an accurate frequency over time. Typically, the more expensive a clock's oscillator, the better the clock's accuracy. Clocks that use cesium or rubidium-based oscillators, for example, can maintain accurate time for long periods after synchronization with a reference time source. Unfortunately, these high-end oscillators are too costly for many applications. Quartz oscillators (ordinary crystal oscillators, TCXOs, OCXOs, etc.), while less expensive, require more frequent synchronization to the reference source to maintain the same accuracy. Many of the environmental effects (on the quartz oscillators) can be minimized by the design of the oscillator and its careful placement in the operating environment. Pressure and humidity effects can be virtually eliminated by sealing the oscillator in a controlled environment (e.g., ovenized enclosure). Most of the remaining effects can be minimized by oscillator placement.